Essentially, this boiled down to two NICS using different DNS servers but why didn’t this affect the SMTP probes to Health Mailboxes? If it didn’t, why did we use local DNS for Health Manager and foreign DNS for delivery? …and, if that’s the case. Is this a bug?

Then, the answer finally arrived in the form of a realization: if we had no queues to the Health Mailboxes, maybe the probe was broken? Alright, time to investigate that piece.

Sure enough, the probe has failed some time back and was marked as poisoned: